3D polysilicon ROM and method of fabrication thereof

ABSTRACT

A 3D polysilicon read only memory at least including: a silicon substrate, an isolated silicon dioxide (SiO 2 ) layer, a N-Type heavily doped (N+) polysilicon layer, a first oxide layer, a dielectric layer, a P-Type lightly doped (P−) polysilicon layer, at least a neck structure, and a second oxide layer. The isolated SiO 2  layer is deposited on the silicon substrate, and the N+ polysilicon layer is deposited on the isolated SiO 2  layer. The N+ polysilicon layer is further defined a plurality of parallel, separate word lines (WL), and the first oxide layer is filled in the space between the word lines. The dielectric layer is deposited on the word lines and the first oxide layer. The P-Type lightly doped (P−) polysilicon layer is deposited on the dielectric layer and is further defined a plurality of parallel, separate bit lines (BL). The bit lines overlap the word lines, from a top view, to form a shape approximately as a cross. There are at least a neck structure individually formed between the first polysilicon layer and the second polysilicon layer by isotropy wet etching the dielectric layer, with using dilute hydrofluoric acid (HF) as the example. The second oxide layer is filled in the space between the bit lines and is on the word lines and the first oxide layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to a polysilicon read only memory (ROM) and method of fabrication thereof, and more particularly to a 3D polysilicon ROM and method of fabrication thereof.

2. Description of the Related Art

3D memories can be much lower cost than conventional 2D memories. If a conventional memory occupies A square millimeters of silicon area, then a 3D memory comprising N planes of bits occupies approximately (A/N) square millimeters of silicon area. Reduced area means that more finished memory devices can be built on a single wafer, thereby reducing cost. Thus there is a strong incentive to pursue 3D memories having many planes of memory cells.

FIG. 1 is a cross-sectional view showing a conventional 3D polysilicon ROM. Referring first to FIG. 1, which is a cross-sectional vies along the direction of bit lines, a conventional 3D polysilicon read only memory (ROM) 10 is a multilayer structure and includes at least: a silicon substrate 110, an isolated silicon dioxide (SiO₂) layer 111, a N-Type heavily doped (N+) polysilicon layer 120, a dielectric layer 130, a P-Type lightly doped (P−) polysilicon layers 140, a oxide layer 124.

The isolated SiO₂ layer 111 is deposited on the silicon substrate 110, and the N-Type heavily doped (N+) polysilicon layer 120 is deposited on the isolated SiO₂ layer 111. The N+ polysilicon layer 120 is further defined a plurality of parallel, separate word lines (WL), such as word lines 122 a, 122 b, 122 c in FIG. 1. The oxide layer 124 is filled in the space between the word lines 122 a, 122 b, 122 c. The dielectric layer 130 is deposited on the word lines 122 a, 122 b, 122 c and on the oxide layer 124.

The P-Type lightly doped (P−) polysilicon layer 140 is deposited on the dielectric layer 130 and is further defined a plurality of parallel, separate bit lines (BL), such as bit lines 142 a, 142 b in FIG. 1. The bit lines 142 a, 142 b overlap the word lines 122 a, 122 b, 122 c, from a top view, to form a shape approximately as a cross.

However, the antifuse breakdown voltage is high between two polysilicon layers in the conventional 3D polysilicon ROM. And, the asymmetrical structure would result in different programming voltage, on current for sense amplifier. In general, the oxide breakdown voltage is high, so it's a key issue to reduce programming voltage. Moreover, the antifuse breakdown is hard to define if the antifuse material is uniform and rough. The antifuse breakdown region is hard to define, so the programming voltages are difficult to be controlled. Hence, the yield in array architectures is low.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide a 3D polysilicon ROM to decrease the required antifuse breakdown voltage which is applied for the occurrence of current breakdown with high electrical breakdown field between two polysilicon layers. And the invention also could well confine the memory cell and well confine the breakdown region so that yield in processing can be improved.

An object of the present invention is to provide a 3D polysilicon ROM at least including: a silicon substrate, an isolated silicon dioxide (SiO₂) layer, a N-Type heavily doped (N+) polysilicon layer, a first oxide layer, a dielectric layer, a P-Type lightly doped (P−) polysilicon layer, at least a neck structures, and a second oxide layer. The isolated SiO₂ layer is deposited on the silicon substrate, and the N+ polysilicon layer is deposited on the isolated SiO₂ layer. The N+ polysilicon layer is further defined a plurality of parallel, separate word lines (WL), and the first oxide layer is filled in the space between the word lines. The dielectric layer is deposited on the word lines and the first oxide layer. The P-Type lightly doped (P−) polysilicon layer is deposited on the dielectric layer and is further defined a plurality of parallel, separate bit lines (BL). The bit lines overlap the word lines, from a top view, to form a shape approximately as a cross. There is at least one neck structure individually formed between the first polysilicon layer and the second polysilicon layer by isotropy wet etching the dielectric layer, with using dilute hydrofluoric acid (HF) as the example. The second oxide layer is filled in the space between the bit lines and is on the word lines and the first oxide layer.

According to another aspect of the present invention, another 3D polysilicon ROM is provided. The 3D polysilicon ROM includes: a silicon substrate, an isolated silicon dioxide (SiO₂) layer, a plurality of word lines (WL), a plurality of bit line (BL) sections, a plurality of dielectric sections, at least a neck structure, a first oxide layer, a plurality of bit lines, a second oxide layer. The isolated SiO₂ layer is deposited on the silicon substrate and there are a plurality of parallel, separate word lines (WL) defined on the isolated SiO₂ layer silicon substrate. A plurality of parallel, separate bit line (BL) sections are formed on the word lines separately. A plurality of parallel, separate dielectric sections are formed below the BL sections one by one, each one being with respect to the BL sections thereon and all being on the word lines. There is at least one neck structure individually formed for the dielectric sections with respect to the bit lines thereon. The first oxide layer is filled in the space between the word lines, in the space between the BL sections, in the space between the dielectric sections, and is on the word lines. A plurality of parallel, separate bit lines are defined on the BL sections and on the first oxide layer. The bit lines overlap the word lines, from a top view, to form a shape approximately as a cross and the bit lines are electrically coupled to the BL sections thereabout. The second oxide layer is filled in the space between the bit lines and is on the first oxide layer over the word lines.

Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (Prior Art) is a cross-sectional view showing a conventional 3D polysilicon ROM.

FIG. 2A is a cross-sectional view showing a 3D polysilicon ROM of the first example of the invention.

FIG. 2B is a flow chart showing the method of fabrication for the 3D polysilicon ROM according to the first example of the invention.

FIG. 2C to 2H are cross-sectional views showing the process steps of the first example of the method of fabrication for the 3D polysilicon ROM.

FIG. 3A is a flow chart showing the method of fabrication for the 3D polysilicon ROM according to the second example of the invention.

FIG. 3B to 3F are cross-sectional views showing the process steps of the second example of the method of fabrication for the 3D polysilicon ROM.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which two examples in accordance with the preferred embodiment of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the examples set forth herein; rather, these examples are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like components throughout.

EXAMPLE 1

FIG. 2A is a cross-sectional view showing a 3D polysilicon ROM of the first example of the invention. Referring first to FIG. 2A, a 3D polysilicon read only memory (ROM) 20 includes a silicon substrate 210, an isolated silicon dioxide (SiO₂) layer 211, a N-Type heavily doped (N+) polysilicon layer 220, a dielectric layer 230, a P-Type lightly doped (P−) polysilicon layers 240, a first oxide layer 224, and a second oxide layer 244.

The isolated SiO₂ layer 211 is deposited on the silicon substrate 210, and the N-Type heavily doped (N+) polysilicon layer 220 is deposited on the isolated SiO₂ layer 211. The N+ polysilicon layer 220 is further defined a plurality of parallel, separate word lines (WL), such as word lines 222 a, 222 b, 222 c in FIG. 2A. The first oxide layer 224 is filled in the space between the word lines 222 a, 222 b, 222 c. The dielectric layer 230 is deposited on the word lines 222 a, 222 b, 222 c and on the first oxide layer 224.

The P-Type lightly doped (P−) polysilicon layer 240 is deposited on the dielectric layer 230 and is further defined a plurality of parallel, separate bit lines (BL), such as bit lines 242 a, 242 b in FIG. 2A. The bit lines 242 a, 242 b overlap the word lines 222 a, 222 b, 222 c, from a top view, to form a shape approximately as a cross.

Moreover, there is at least one neck structure, such as neck 231 a, 231 b, individually formed between the N-Type heavily doped (N+) polysilicon layer 220 and the P-Type lightly doped (P−) polysilicon layer 240 by isotropy etching the dielectric layer. The neck 231 a, 231 b are under the bit lines 242 a, 242 b, respectively. The second oxide layer 244 is filled in the space between the bit lines 242 a, 242 b and is on the word lines 222 a, 222 b, 222 c and on the first oxide layer 224.

FIG. 2B is a flow chart showing the method of fabrication for the 3D polysilicon ROM according to the first example of the invention, and FIG. 2C to 2H are cross-sectional views showing the process steps of the first embodiment of the method of fabrication for the 3D polysilicon ROM. Referring first to FIG. 2B, as described in step 261, a substrate 210 is provided. Then, as described in step 263, an isolated SiO₂ layer 211 is deposited on the substrate 210. As described in step 265, a N-Type heavily doped (N+) polysilicon layer 220 is formed on the isolated SiO₂ layer 211; the N-Type heavily doped (N+) polysilicon layer is further patterned to define a plurality of parallel, separate word lines (WL) 222 a, 222 b, 222 c, as shown in FIG. 2C.

As described in step 267, the space between the word lines 222 a, 222 b, 222 c is filled to form a first oxide layer 224, as shown in FIG. 2D. And then, as described in step 269, the N-Type heavily doped (N+) polysilicon layer 220 and the first oxide layer 224 are planarized to form a planarized surface. In FIG. 2E, as described in step 271, a dielectric layer 230 is formed on the planarized surface, i.e. the dielectric layer 230 is formed on the word lines 222 a, 222 b, 222 c, and on the first oxide layer 224.

As described in step 273, a P-Type lightly doped (P−) polysilicon layer 240 is formed on the dielectric layer 230; the P-Type lightly doped (P−) polysilicon layer 240 is patterned to define a plurality of parallel, separate bit lines (BL) 242 a, 242 b. The bit lines 242 a, 242 b overlap the word lines 222 a, 222 b, 222 c, from a top view, to form a shape approximately as a cross, as shown in FIG. 2F.

Then, as described in step 275, the dielectric layer 230 is isotropy etched by wet etching, with preferably using dilute hydrofluoric acid (HF) here as the example, to form two continuing narrow necks 231 a, 231 b between the N-Type heavily doped (N+) polysilicon layer 220 and the P-Type lightly doped (P−) polysilicon layer, as shown in FIG. 2G. Finally, as described in step 277, a second oxide layer 244 is formed by filling oxides in the space between the bit lines 242 a, 242 b and is on the word lines 222 a, 222 b, 222 c and the first oxide layer 224, as shown in FIG. 2H.

The present inventions of the first example are not limited in what are described above. For example, user can repeat the process steps in accordance with the method discloses in FIG. 2B and make multi-layer stacks to meet their demands. Moreover, the dielectric layer 230 preferably can be made of silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂) or zirconium oxide (ZrO₂). Also, the different etching solutions are used with respect to the concerned material of the dielectric layer 230, such as dilute hydrofluoric acid (HF) used in the first example of the invention. In addition, the N-Type heavily doped (N+) polysilicon layer 220 and the P-Type lightly doped (P−) polysilicon layers 240, both can be replaced by a sandwich structure of polysilicon/silicide/polysilicon in order to reduce the corresponding resistance. The oxide layer 222,224 can be filled by high density plasma (HDP) in a blanket deposition in the space between the bit lines 222 a, 222 b, 222 c, and between the word lines 242 a, 242 b, respectively. Moreover, the oxide layer 222,224 can be made of silicon nitride (Si₃N₄), Borophosphosilicate glass (BPSG), a polymer or a low K material.

EXAMPLE 2

FIG. 3A is a flow chart showing the method of fabrication for the 3D polysilicon ROM according to the second embodiment of the invention, and FIG. 3B to 3F are cross-sectional views showing the process steps of the second embodiment of the method of fabrication for the 3D polysilicon ROM. Referring first to FIG. 3F, a 3D polysilicon read only memory (ROM) 30 includes a silicon substrate 310, an isolated silicon dioxide (SiO₂) layer 311, a N-Type heavily doped (N+) polysilicon layer 320, a dielectric layer 330, P-Type lightly doped (P−) polysilicon layers 340, 350, and oxide layers 344,354.

The method of fabrication for the 3D polysilicon ROM according to the second embodiment of the invention is described below. First, as described in step 361, a silicon substrate 310 is provided. Then, as described in step 363, an isolated SiO₂ layer 311 is deposited on the substrate 310. As described in step 365, a N-Type heavily doped (N+) polysilicon layer 320 is formed on the isolated SiO₂ layer 311. Then, as described in step 367, a dielectric layer 330 is formed on the N-Type heavily doped (N+) polysilicon layer 320.

As described in step 369, a P-Type lightly doped (P−) polysilicon layer 340 is formed on the dielectric layer 330; the P-Type lightly doped (P−) polysilicon layer 340 is patterned to define a plurality of parallel, separate first bit lines (BL) 342 a, 342 b, 342 c. Moreover, the dielectric layer 330 is further patterned to define a plurality of parallel, separate dielectric rails 332 a, 332 b, 332 c, and the dielectric rails 332 a, 332 b, 332 c are formed below the bit lines 342 a, 342 b, 342 c individually, as shown in FIG. 3B.

Further, as described in step 371, a plurality of parallel, separate bit line (BL) sections are formed for each first bit line, and a plurality of parallel, separate dielectric sections are formed, i.e. BL sections 342 a 1, 342 b 1 and dielectric sections 332 a 1, 332 b 1 are formed for bit line 342 a and for the dielectric rail 332 a. BL sections 342 a 2, 342 b 2 and dielectric sections 332 a 2, 332 b 2 are formed for bit line 342 b and for the dielectric rail 332 b. BL sections 342 a 3, 242 b 3 and dielectric sections 332 a 3, 332 b 3 are formed for bit line 342 c and for the dielectric rail 332 c. The dielectric sections 332 a 1, 332 a 2, 332 b 1, 332 b 2, 332 c 1, 332 c 2 are formed below the BL sections 342 a 1, 242 b 1, 342 a 2, 342 b 2, 342 a 3, 342 b 3, and each one dielectric section is with respect to the BL sections thereon and all dielectric sections are on the word lines 332 a, 332 b, as shown in FIG. 3C.

As described in step 373, the dielectric sections 332 a 1, 332 a 2, 332 a 3, 332 b 1, 332 b 2, 332 b 3 are isotropy etched by wet etching, with preferably using dilute hydrofluoric acid (HF) here as the example, to form an isolated neck along the word line direction and to form another isolated neck along the bit line direction between the N-Type heavily doped (N+) polysilicon layer 320 and the P-Type lightly doped (P−) polysilicon layer 340, so that the dielectric sections 332 a 1′, 332 a 2′, 332 a 3′, 332 b 1′, 332 b 2′, 332 b 3′. The dielectric sections 332 a 1′, 332 a 2′, 332 a 3′, 332 b 1′, 332 b 2′, 332 b 3′ are located with respect to the BL sections 342 a 1, 342 a 2, 342 b 1, 342 b 2, 342 a 3, 342 b 3, as shown in FIG. 3D.

As described in step 375, a first oxide layer 344 is formed by filling in the space between the word lines 322 a, 322 b, in the space between the BL sections 342 a 1, 342 a 2, 342 b 1, 342 b 2, 342 a 3, 342 b 3, in the space between the dielectric sections 332 a 1′, 332 a 2′, 332 a 3′, 332 b 1′, 332 b 2′, 332 b 3′, and is on the word lines 322 a, 322 b, as shown in FIG. 3E.

As described in step 377, a plurality of parallel, separate second bit lines are formed on the BL sections and on the first oxide layer, i.e. the second bit line 352 a 1 is formed on the BL sections 342 a 1, 342 b 1 and on the first oxide layer 344 and the second bit line 352 a 1 is electrically coupled to the BL sections 342 a 1, 342 b 1. The second bit line 352 a 2 is formed on the BL sections 342 a 2, 342 b 2 and on the first oxide layer 344, and the second bit line 352 a 2 is electrically coupled to the BL sections 342 a 2, 342 b 2. The second bit line 352 a 3 is formed on the BL sections 342 a 3, 342 b 3 and on the first oxide layer 344, and the second bit line 352 a 3 is electrically coupled to the BL sections 342 a 3, 342 b 3. The second bit lines 352 a 1, 352 a 2, 352 a 3 overlap the word lines 322 a, 322 b, from a top view, to form a shape approximately as a cross. Finally, as described in step 379, a second oxide layer 354 is formed by filling oxides in the space between the second bit lines 352 a 1, 352 a 2, 352 a 3 and is on the first oxide layer over the word lines, as shown in FIG. 3F.

The present inventions of the second example are not limited in what are described above. For example, user can repeat the process steps in accordance with the method discloses in FIG. 3A and make multi-layer stacks to meet their demands. Moreover, the dielectric layer 330 preferably can be made of silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂) or zirconium oxide (ZrO₂). Also, the different etching solutions are used with respect to the concerned material of the dielectric layer 230, such as dilute hydrofluoric acid (HF) used in the second example of the invention. In addition, the N-Type heavily doped (N+) polysilicon layer 320 and the P-Type lightly doped (P−) polysilicon layers 340,350 all can be replaced by a sandwich structure of polysilicon/silicide/polysilicon in order to reduce the corresponding resistance. The oxide layer 344,354 can be filled by high density plasma (HDP) in a blanket deposition in the space between the BL sections 342 a 1, 342 a 2, 342 a, 342 b 1, 342 b 2, 342 b 3, the dielectric sections 332 a 1′, 332 a 2′, 332 a 3′, 332 b 1′, 332 b 2′, 332 b 3′, and between the bit lines 352 a 1, 352 a 2, 352 a 3, respectively. Moreover, the oxide layer 344,354 can be made of silicon nitride (Si₃N₄), Borophosphosilicate glass (BPSG), a polymer or a low K material.

In summary, the invention achieves the above-identified object by providing an improved 3D polysilicon ROM which is fabricated by wet etching, with preferably using dilute hydrofluoric acid (HF) as the example, to form a neck (undercut) structure between two polysilicon layers. The advantages of the neck structure are able to decrease the required antifuse breakdown voltage, to well confine the memory cell and well confine the breakdown region so that yield in processing can be improved.

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 

1. A three dimension (3D) polysilicon read only memory (ROM), at least comprising: a silicon substrate; an isolated silicon dioxide (SiO₂) layer, which is deposited on the silicon substrate; a N-Type heavily doped (N+) polysilicon layer, which is deposited on the isolated SiO₂ layer and is further defined a plurality of parallel, separate word lines (WL); a first oxide layer, which is filled in the space between the word lines; a dielectric layer, which is deposited on the word lines and the first oxide layer; a P-Type lightly doped (P−) polysilicon layer, which is deposited on the dielectric layer and is further defined a plurality of parallel, separate bit lines (BL), wherein the bit lines overlap the word lines, from a top view, to form a shape approximately as a cross; at least a neck structure, which are individually formed between the first polysilicon layer and the second polysilicon layer by isotropy etching the dielectric layer and using dilute hydrofluoric acid (HF); and a second oxide layer, which is filled in the space between the bit lines and is on the word lines and the first oxide layer.
 2. The 3D polysilicon ROM according to claim 1, wherein the N-Type heavily doped (N+) polysilicon layer is a sandwich structure of polysilicon/silicide/polysilicon.
 3. The 3D polysilicon ROM according to claim 1, wherein the P-Type lightly doped (P−) polysilicon layer is a sandwich structure of polysilicon/silicide/polysilicon.
 4. The 3D polysilicon ROM according to claim 1, wherein the first oxide layer and the second oxide layer are filled by high density plasma in the space between the word lines and between the bit lines, respectively.
 5. The 3D polysilicon ROM according to claim 1, wherein the first oxide layer and the second oxide layer are made of silicon nitride (Si₃N₄).
 6. The 3D polysilicon ROM according to claim 1, wherein the first oxide layer and the second oxide layer are made of Borophosphosilicate glass (BPSG).
 7. The 3D polysilicon ROM according to claim 1, wherein the first oxide layer and the second oxide layer are made of a polymer.
 8. The 3D polysilicon ROM according to claim 1, wherein the first oxide layer and the second oxide layer are made of a low K material.
 9. The 3D polysilicon ROM according to claim 1, wherein the material of the dielectric layer is selected from a group consisting of silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂) and zirconium oxide (ZrO₂).
 10. A method of fabricating a 3D polysilicon ROM, comprising the steps of: providing a substrate; depositing an isolated SiO₂ layer on the substrate; forming a first polysilicon layer on the isolated SiO₂ layer; patterning the first polysilicon layer to define a plurality of parallel, separate word lines (WL); filling the space between the word lines to form a first oxide layer; planarizing the first polysilicon layer and the first oxide layer to form a planarized surface; forming a dielectric layer on the planarized surface; forming a second polysilicon layer on the dielectric layer; patterning the second polysilicon layer to define a plurality of parallel, separate bit lines (BL), wherein the bit lines overlap the word lines, from a top view, to form a shape approximately as a cross; isotropy etching the dielectric layer by wet etching, with using dilute hydrofluoric acid (HF), to form necks between the first polysilicon layer and the second polysilicon layer; and forming a second oxide layer by filling oxides in the space between the bit lines and is on the word lines and the first oxide layer.
 11. The method of fabricating a 3D polysilicon ROM according to claim 10, the first polysilicon layer is a N-Type heavily doped (N+) polysilicon layer, and the second polysilicon layer is a P-Type lightly doped (P−) polysilicon layer.
 12. The method of fabricating a 3D polysilicon ROM according to claim 11, wherein the N-Type heavily doped (N+) polysilicon layer is a sandwich structure of polysilicon/silicide/polysilicon.
 13. The method of fabricating a 3D polysilicon ROM according to claim 11, the P-Type lightly doped (P−) polysilicon layer is a sandwich structure of polysilicon/silicide/polysilicon.
 14. The method of fabricating a 3D polysilicon ROM according to claim 10, wherein the first oxide layer and the second oxide layer are filled by high density plasma in the space between the word lines and between the bit lines, respectively.
 15. The method of fabricating a 3D polysilicon ROM according to claim 10, wherein the first oxide layer and the second oxide layer are made of silicon nitride (Si₃N₄).
 16. The method of fabricating a 3D polysilicon ROM according to claim 10, wherein the first oxide layer and the second oxide layer are made of Borophosphosilicate glass (BPSG).
 17. The method of fabricating a 3D polysilicon ROM according to claim 10, wherein the first oxide layer and the second oxide layer are made of a polymer.
 18. The method of fabricating a 3D polysilicon ROM according to claim 10, wherein the first oxide layer and the second oxide layer are made of a low K material.
 19. The method of fabricating a 3D polysilicon ROM according to claim 10, wherein the material of the dielectric layer is selected from a group consisting of silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂) and zirconium oxide (ZrO₂).
 20. A 3D polysilicon ROM, at least comprising: a silicon substrate; depositing an isolated SiO₂ layer on the silicon substrate; a plurality of parallel, separate word lines (WL), which are defined on the isolated SiO₂ layer; a plurality of parallel, separate bit line (BL) sections, which are formed on the word lines separately; a plurality of parallel, separate dielectric sections, which are formed below the BL sections one by one, each one being with respect to the BL sections thereon and all being on the word lines; at least a neck structure, which are individually formed for the dielectric sections with respect to the bit lines thereon; a first oxide layer, which is filled in the space between the word lines, in the space between the BL sections, in the space between the dielectric sections, and is on the word lines; a plurality of parallel, separate bit lines, which are defined on the BL sections and on the first oxide layer, wherein the bit lines overlap the word lines, from a top view, to form a shape approximately as a cross and the bit lines are electrically coupled to the BL sections thereabout; and a second oxide layer, which is filled in the space between the bit lines and is on the first oxide layer over the word lines.
 21. The 3D polysilicon ROM according to claim 20, wherein the first oxide layer and the second oxide layer are filled by high density plasma in the space between the word lines and between the bit lines, respectively.
 22. The 3D polysilicon ROM according to claim 20, wherein the first oxide layer and the second oxide layer are made of silicon nitride (Si₃N₄).
 23. The 3D polysilicon ROM according to claim 20, wherein the first oxide layer and the second oxide layer are made of Borophosphosilicate glass (BPSG).
 24. The 3D polysilicon ROM according to claim 20, wherein the first oxide layer and the second oxide layer are made of a polymer.
 25. The 3D polysilicon ROM according to claim 20, wherein the first oxide layer and the second oxide layer are made of a low K material.
 26. The 3D polysilicon ROM according to claim 20, wherein the material of the dielectric layer is selected from a group consisting of silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂) and zirconium oxide (ZrO₂).
 27. A method of fabricating a 3D polysilicon ROM, comprising the steps of: providing a silicon substrate; forming an isolated SiO₂ layer on the silicon substrate; forming a first polysilicon layer on the isolated SiO₂ layer; forming a dielectric layer on the first polysilicon layer; forming a second polysilicon layer on the dielectric layer; patterning the second polysilicon layer to define a plurality of parallel, separate first bit lines (BL) and patterning the dielectric layer to define a plurality of parallel, separate dielectric rails, wherein the dielectric rails are formed below the first bit lines; forming a plurality of parallel, separate bit line (BL) sections for each first bit line and forming a plurality of parallel, separate dielectric sections, wherein the dielectric sections are formed below the BL sections one by one, each one being with respect to the BL sections thereon and all being on the word lines; isotropy etching the dielectric sections by wet etching, with using dilute hydrofluoric acid (HF), to form necks between the first polysilicon layer and the second polysilicon layer; filling in the space between the word lines, in the space between the BL sections, in the space between the dielectric sections, and is on the word lines, to form a first oxide layer; forming a plurality of parallel, separate second bit lines on the BL sections and on the first oxide layer, wherein the second bit lines overlap the word lines, from a top view, to form a shape approximately as a cross and the second bit lines are electrically coupled to the BL sections thereabout; and forming a second oxide layer by filling oxides in the space between the second bit lines and is on the first oxide layer over the word lines.
 28. The method of fabricating a 3D polysilicon ROM according to claim 27, the first polysilicon layer is a N-Type heavily doped (N+) polysilicon layer, and the second polysilicon layer is a P-Type lightly doped (P−) polysilicon layer.
 29. The method of fabricating a 3D polysilicon ROM according to claim 28, wherein the N-Type heavily doped (N+) polysilicon layer is a sandwich structure of polysilicon/silicide/polysilicon.
 30. The method of fabricating a 3D polysilicon ROM according to claim 28, the P-Type lightly doped (P−) polysilicon layer is a sandwich structure of polysilicon/silicide/polysilicon.
 31. The method of fabricating a 3D polysilicon ROM according to claim 27, wherein the first oxide layer and the second oxide layer are filled by high density plasma in the space between the word lines and between the bit lines, respectively.
 32. The method of fabricating a 3D polysilicon ROM according to claim 27, wherein the first oxide layer and the second oxide layer are made of silicon nitride (Si₃N₄).
 33. The method of fabricating a 3D polysilicon ROM according to claim 27, wherein the first oxide layer and the second oxide layer are made of Borophosphosilicate glass (BPSG).
 34. The method of fabricating a 3D polysilicon ROM according to claim 27, wherein the first oxide layer and the second oxide layer are made of a polymer.
 35. The method of fabricating a 3D polysilicon ROM according to claim 27, wherein the first oxide layer and the second oxide layer are made of a low K material.
 36. The method of fabricating a 3D polysilicon ROM according to claim 27, wherein the material of the dielectric layer is selected from a group consisting of silicon dioxide (SiO₂), silicon nitride (Si₃N₄), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂) and zirconium oxide (ZrO₂). 